1. Field of the Invention
This invention relates to a process for depositing titanium oxide and tin oxide coatings on a flat glass substrate, and the resulting coated glass. More particularly, this invention relates to a chemical vapour deposition process for producing titanium oxide and tin oxide coatings on flat glass using a coating precursor gas mixture comprising the corresponding metal tetrachloride and an organic oxidant.
2. Summary of Related Art
Titanium oxide and tin oxide coatings have been proposed for use on glass containers, for example bottles, to improve the mechanical strength of the containers. It has also been proposed to use both titanium oxide and tin oxide coatings on flat glass to modify the characteristics of the glass for architectural use; titanium oxide coatings deposited under vacuum (by reactive sputtering) are used as components of sputtered multi-layer infrared reflecting coatings, while tin oxide coatings are used, not only as layers of multi-layer sputtered coatings, but also deposited pyrolytically with a dopant as infrared reflecting and/or electroconducting coatings.
GB patent specification 1 115 342 describes a process for producing glass containers with good inherent strength and good abrasion resistance by spraying the containers, while still hot from the manufacturing process, with a solution or dispersion of stannic chloride (that is, tin tetrachloride) in an organic liquid, isopropyl alcohol being preferred. A small amount of titanium chloride may be incorporated as a modifier. The liquid solution is fed to atomisers, which may be of the pressure jet variety, located on either side of a tunnel over a conveyor for hot glass bottles to produce a `mist of liquid reagent` so that a layer of liquid is formed on all the external surfaces of the bottles where it reacts to form a layer of tin oxide.
GB patent specification 1 187 784 describes an improvement of the process described in GB patent specification 1 115 342 and apparently more suitable for incorporation into a process for the automatic manufacture of glassware without interfering with the normal running of such process and without requiring additional supervision. The specification proposes to treat glass containers, at high temperature, with a liquid solution of an organic compound of tin "which compound has properties such that upon application of heat it decomposes into two materials, one of which is an organic compound of tin of high decomposition temperature which reacts with the glass surface to produce a diffusion layer of tin oxide within the glass surface, while the other is a volatile compound of tin such that a substantial proportion of vapour of said compound is produced, and subjecting the containers to a heat treatment such that a reaction is caused to occur between the glass at least the surfaces of the containers and the tin compounds". The material used for treating the glass containers may be provided by reacting tin tetrachloride with organic substances containing carbonyl groups of moderate activity e.g. organic esters of ethyl, n-propyl, isopropyl, n-butyl and isobutyl alcohols with acetic, propionic and butyric acids. The resulting solution may be sprayed, in the presence of ambient atmosphere, on to the hot containers e.g. in the form of a fine mist after they leave the forming machine and before they enter the annealing lehr. GB patent specification 1 187 783 describes an analogous process to that described in 1 187 784 in which an organic compound of titanium is sprayed on to the hot glass containers in place of the organic compound of tin. The organic titanium compound may be produced, in an analogous manner to the organic compound of tin, by reacting titanium tetrachloride with an organic ester e.g. n-butyl acetate. Again, the resulting solution is sprayed onto the glass in the ambient atmosphere on the container production line.
It has also been proposed to use tin tetrachloride, applied either as a liquid spray or, more recently, in gaseous form, to apply a tin oxide coating to hot flat glass to form an electroconductive, infra-red reflecting coating on the hot glass surface; water is used to hydrolyse the tin tetrachloride and as a source of oxygen for formation of the tin oxide.
Processes involving use of the reactants in gaseous form (also called CVD or chemical vapour deposition processes) have certain advantages over spray processes for coating flat glass, especially when the reactants can be premixed before application to the glass. Unfortunately, tin tetrachloride reacts readily with water so that previous proposals to use tin tetrachloride and water vapour in gaseous form have usually involved supplying the gases separately to the glass surface and mixing them while in contact with the glass.
GB patent specification 2 044 137A relates to such a process in which discrete laminar streams of each reactant are formed and projected on to a hot glass substrate by bringing the streams together in reciprocal tangential contact over the glass. Titanium tetrachloride may be used as one of the gaseous reactants, in place of the tin tetrachloride, in order to form a titanium oxide coating. The patent also suggests supplying hydrogen to one of the gas streams to attenuate the violent reaction between the tin tetrachloride and the water vapour. This may be done either by direct addition of gaseous hydrogen, or by the addition of methanol, which is said to react in situ to produce the desired gaseous hydrogen.
GB patent specification 2 026 454B describes a process in which a coating chamber is positioned over a hot float glass ribbon as it advances from the float bath and successive gaseous streams of (1) preheated nitrogen carrier gas, (2) tin tetrachloride entrained in preheated nitrogen and (3) air, water vapour and hydrofluoric acid are introduced into the coating chamber so they flow along the glass substrate surface being coated as a substantially turbulence free layer. The patent specifies the concentration of water vapour and tin tetrachloride in the gaseous medium over the glass.
European patent specifications 0 365 239B1 and 0 376 240B1 describe a method and apparatus for depositing a tin oxide coating on a hot glass ribbon. A first gaseous stream of tin tetrachloride in preheated dry air is caused to flow along the surface of the hot ribbon of glass advancing beneath a coating chamber, a second turbulent stream of hydrofluoric acid and steam introduced into the coating chambers at right angles to the plane of the glass and direction of flow of the first gaseous stream, and the combined first and second gas streams drawn through the coating chamber over the glass under turbulent flow conditions. The method and apparatus may also be used to apply a coating of titanium oxide using titanium tetrachloride in place of the tin tetrachloride.
U.S. Pat. No. 4,590,096 describes a process in which a coating solution comprising a substantially solvent free mixture of an organotin chloride and a reactive organic fluorine compound soluble in or miscible with the organotin chloride is introduced to a preheated carrier gas stream which contains sufficient water vapour that the relative humidity of the gas stream at 18.degree. C. is about 6% to about 100%. The resulting gas stream is passed over a hot glass surface to deposit a fluorine doped tin oxide coating on the hot glass. A wide range of organotin compounds may be used, and the possibility of using tin tetrachloride is mentioned. Similarly, a wide range of organic fluorine compounds, including oxygen containing compounds, for example trifluoroacetic acid and ethyltrifluoroacetate, may be used. Some of the fluorine-containing dopants have limited solubilities in the organotin compounds used, and an optional solubiliser may be used to increase the solubility of the fluorine dopant on the organotin compound; acetic anhydride, ethyl acetate, hexane, methyl isobutyl ketone and butyraldehyde are listed as non-limiting examples of the solubilisers that may be used. However, the U.S. patent, in common with the other patents utilising chemical vapour deposition methods to deposit a metal oxide from a gaseous metal tetrachloride, utilises water vapour as the source of oxygen.
U.S. Pat. No. 4,751,149 Vijaykumar et al is concerned with deposition of zinc oxide coatings by chemical vapour deposition at low temperature (600 to 350.degree. C., preferably 100.degree. to 200.degree. C.) on heat sensitive photoconductor substrates, and proposes to deposit the zinc oxide coatings from an organozinc compound and an oxidant, which may be an oxygen containing organic compound e.g. an ester, and an inert carrier gas. Although the patent is not entirely clear, it apparently proposes to introduce separate streams of the organozinc compound and oxidant into the deposition chamber, and certainly there is no proposal to pre-mix these components together before delivery to the coating chamber.
It would be advantageous to provide a method for depositing tin or titanium oxide coatings by a CVD process applied to hot flat glass using a premixture of the corresponding metal tetrachloride as a low cost reactant and a source of oxygen without premature reaction between the metal tetrachloride and oxygen source (previously water) resulting in formation of metal oxide in the coating equipment with consequent problems and inefficiency. It would be particularly advantageous if the method allowed for deposition of the coating at high rates.
It is an object of the present invention to provide a method for obtaining tin and titanium oxide coatings on a substrate at high deposition rates. High deposition rates are important when coating substrates in a manufacturing process. This is particularly true for an on-line float glass process where the glass ribbon is traveling at a specific line speed and where the ribbon requires a specific coating thickness. The deposition rates obtained with the preferred embodiments of the present invention may be ten times higher than the deposition rates with other known methods for depositing titanium oxide coatings. Especially high deposition rates, particularly for titanium oxide, may be achieved with the present invention using a precursor mixture including an ester with a .beta. hydrogen.
Another object of the present invention is to provide a method for obtaining a tin oxide or titanium oxide coating wherein the thickness of the coating can be varied based upon the particular organic compound utilized as a source of oxygen in the precursor mixture.
A further object is to obtain a titanium oxide coating at high deposition rates with a refractive index of at least 2.4.
A still further object is to produce tin and titanium oxide coatings at high deposition rates using low cost chlorinated precursors.